Such an optical fiber is, in particular, suitable for making amplifiers for transoceanic transmission systems using wavelength division multiplexing (WDM). Optical fiber transmission systems, and in particular transoceanic systems, use amplification devices at regular intervals, each amplification device being formed by a fiber doped with a rare earth such as erbium, and by a pump. In the specific case of WDM transmission, the amplification devices should have a bandwidth at least equal to that of the multiplex, and a gain peak as flat as possible centered on the band of the multiplex.
These characteristics may be obtained by adding filters, e.g. Bragg gratings, as described in O. Gautheron, P. Sansonnetti, G. Bassier, and I Riant, "Optical gain equalisation with short period fiber gratings", ECOC'97, pages 131-134, 1997, or else in N. Bergano et al., "100 Gb/s WDM transmission of twenty 5 Gb/s NRZ data channels over transoceanic distances using a gain flattened amplifier chain", ECOC'95, pages 967-970, 1995. It would however be advantageous to obtain these characteristics by means of the amplification fiber itself.
To that end, proposals have already been made to associate two different fiber sections mounted one after the other in the amplifier. C. R. Giles and D. DiGiovanni, "Dynamic Equalization in Two-Stage Amplifiers", OAA'90, MD2, p. 48-51, proposes to correct the relative gain variations between the channels of a wavelength division multiplex by means of a two-stage amplifier, in which the two stages have complementary gain spectra. That document proposes a first stage formed by an erbium-doped fiber having a germano-alumino-silicate core, and a second stage formed by an erbium-doped fiber having an alumino-silicate core. By pumping the two fibers separately, it is possible to equalize the gain over the range of the multiplex.
T. Kashiwada et al., "Gain-flattened optical-fiber amplifiers with a hybrid Er-doped-fiber configuration for WDM transmission", OFC'95 Technical Digest, TuP1, pp. 77-78, proposes an erbium-doped fiber amplifier formed by a section of fiber co-doped with aluminum (1.4% by weight) cascaded with a section of fiber co-doped with aluminum and with phosphorus (5% by weight of P and 1% by weight of Al). That document proposes using a single pump, but equalizing the gain by an appropriate choice of fiber section lengths.
M. Kakui et al., "Low noise, high power optical amplifiers employing an improved hybrid Er-doped fiber configuration for WDM transmission", OAA'96, Technical Digest, SaA3, pp. 203-206 proposes a two-stage amplifier; the first stage is formed of an erbium-doped fiber co-doped with aluminum; the second stage is formed of an erbium-doped fiber section co-doped with aluminum and with phosphorus and cascaded with an erbium-doped fiber section co-doped with aluminum. Each section is pumped separately, and the choice of the relative lengths of the fibers makes it possible to select the input power level for which gain equalization is maximized.
M. Kakui et al., "Design Optimization of Hybrid Erbium-Doped Fiber Amplifiers for WDM Transmission Systems", Optical Fiber Technology 3 (1997), pp. 123-133, proposes cascading an erbium-doped fiber co-doped with aluminum and an erbium-doped fiber section co-doped with phosphorus and with aluminum; in the same way, the relative lengths of the fiber sections make it possible to equalize the gain.
P. F. Wysocki and D. DiGiovanni, "Dual-Stage Erbium-Doped, Erbium/Ytterbium Codoped Fiber Amplifier with up to +26 dBm Output and 17 nm Flat Spectrum", OAA'96, Technical Digest 1996, SaA2, proposes an amplifier with a first fiber section doped with erbium and co-doped with aluminum, and a second fiber section doped with erbium and with ytterbium, and co-doped with phosphorus. The two sections are pumped independently, and the gain is equalized by appropriately choosing respective lengths for the two sections.
P. Nilson et al., "Simple gain-flattened erbium-doped fiber amplifier with a wide dynamic range", OFC'97 Technical Digest, pp. 129-130, proposes a first fiber section doped with erbium and co-doped with phosphorus, and a second fiber section doped with erbium and co-doped with aluminum and with germanium. The resulting amplifier presents gain flatness over a dynamic range of 15 dB by means of an appropriate choice of the co-dopants and of the fiber lengths.
Those solutions suffer from the drawback of increasing the cost of the amplifiers because of the existence of various components and because of the need to assemble together various fiber sections, usually together with an isolator.
Proposals have also been made to use co-dopants such as aluminum and phosphorus in the same fiber. B. J. Ainslie et al., "Erbium doped fibers for efficient optical amplifiers", IEE Proceedings, vol. 137, Pt. J. No. 4, August 1990, describes using an erbium-doped A1.sub.2 O.sub.3 --P.sub.2 O.sub.5 --SiO.sub.2 fiber for optical amplifiers. That document makes no mention of the gain equalization problem, and suggests no solution to this problem.
B. M. Desthieux et al, "Enhanced spectral gain-response of in-line amplifiers for transoceanic WDM systems using phosphorus aluminum codoped EDFAs", OAA'96 Technical Digest, SaA4-1, pp. 207-210, compares an erbium-doped and P.sub.2 O.sub.5 --Al.sub.2 O.sub.3 co-doped fiber with Al.sub.2 O.sub.3 --GeO.sub.2 co-doped fiber. That document makes no mention of the proportions of the co-dopants or of the positioning of the dopants in the fiber.
EP-A-0 602 467 describes an erbium-doped fiber for a fiber amplifier. The aim pursued in that document is to increase the numerical aperture of the fiber; for that purpose, it is proposed to dope the fiber core with Al.sub.2 O.sub.3, GeO.sub.2 or P.sub.2 O.sub.5 ; it is also proposed to provide a stress-reducing layer between the fiber core and the cladding, so as to reduce the differences between the coefficients of thermal expansion. That layer may be doped with P.sub.2 O.sub.5 and F.sub.2. It is also possible to provide a barrier layer between the fiber core and the stress-reducing layer, so as to limit the diffusion of phosphorus into the core. That also limits the diffusion of erbium into the stress-reducing layer.
FR-A-2 740 563 proposes a silica optical fiber whose core comprises a central zone containing erbium, aluminum, and optionally germanium, a low-doped or non-doped intermediate zone, and a peripheral zone doped with germanium or with phosphorus. The low-doped or non-doped intermediate zone acts as a barrier against the diffusion of erbium into the peripheral zone.
Those various documents do not provide any solution to the problem of equalizing gain in erbium-doped fiber amplifiers for WDM transmission systems.
A. Bjarklev, "Hybrid Erbium-Doped Fiber for Gain Flattened Operation", Optical Fiber Technology 3 (1997), pp. 72-76, proposes an optical fiber whose erbium-doped core (of radius r.sub.2 =a=1.625 m) comprises a central zone (of radius r.sub.1 =0.5 a) co-doped with Al and a peripheral zone co-doped with Ge, together with a second optical fiber which is similar but in which the zones in which the co-dopants are placed are reversed relative to the first fiber (and with a radius r.sub.1 =0.65 a). That article emphasizes how difficult it is to manufacture erbium-doped fibers having two different co-dopants, because of the micro-structures in which the erbium ions are incorporated during fiber manufacture.
D. J. Di Giovanni et al., "Structure and properties of silica containing aluminum and phosphorus near the AlPO.sub.4 join", J. of Non-Crystalline Solids 113, pp. 58-64, 1989, explains that mixing Al and P in a silica matrix causes AlPO.sub.4 to be formed, which has the effect of decreasing the refractive index of the fiber. That article suggests associating Al and P so as to obtain glasses having new properties.